NOTES. 1/5 



metry, and the recent researches of E. B. WILSON and LWOFF lead 

 to the conclusion that the so-called mesoblastic pole-cells, which 

 were described by HATSCHEK, have no real independent existence. 



3. (p. 123.) Whether the dorsal and ventral fin-spaces are 

 actually derived from the original myoccel, as described by Hat- 

 schek, or do not rather arise by a splitting of an originally solid 

 thickening of the gelatinous connective tissue which surrounds 

 them, must remain doubtful. The cavity of the metapleural folds 

 certainly arises as a schizocwl, i.e. by a hollowing out of a solid 

 thickening. Even in case the fin-spaces also arise as schizoccels, 

 Hatschek's interpretation of their morphological significance might 

 still hold good. 



4. (p. 123.) A transitory pouch-like diverticulum of the myo- 

 ccel has been observed in connexion with the formation of the 

 sclerotome in the Selachian embryo by RABL and H. E. ZIEGLER. 



5. (p. 129.) Since the work of BALFOUR on the development 

 of Elasmobranch fishes (Selachians), it has been known that the 

 paired praemandibular head-cavities communicate with one another 

 across the median line in the embryo. The important results 

 obtained by the researches of KUPFFER (Petromyzon, Acipenser), 

 KASTSCHENKO (Selachian), and JULIA PLATT (Selachian), not only 

 established the fact that the prsemandibular cavities arose essen- 

 tially as anterior archenteric pouches (cf. Fig. 72), but also that 

 the median cavity which effected their communication across the 

 middle line, from side to side, arose by constriction from the front 

 end of the archenteron (using the latter term with some latitude), 

 and that, therefore, the union of the right and left prcemandibular 

 cavities in the embrvo of the craniate Vertebrates is primary, and 

 not secondary, as was previously supposed. 



For an excellent historical and critical summary of our knowl- 

 edge of the origin of the head-cavities in the craniate Vertebrates, 

 the reader may consult FRORIEP. (See bibliography.) 



6. (p. 130.) The ciliation of the ectoderm in the larva of 

 Amphioxus continuing, as it does, long after the muscles have been 

 fully differentiated, and when the cilia are therefore no longer 

 required for purposes of locomotion, should be especially noted as 

 evidence of a very archaic organisation. 



We shall find in the last chapter that the possession of a ciliated 



